Big Bang Nucleosynthesis

Figure 12 : A graph showing the fractional composition (left axis) of
the Universe during the first few minutes after the Big Bang, when
nuclear reactions were raging. As time passes (top axis) the
temperature of the Universe (bottom axis) drops from 3 billion degrees
to a hundred million degrees. After about 20 seconds, helium starts
being produced, along with smaller amounts of other light elements
(Deuterium, Beryllium, Lithium). The figure was taken from Ed Wrights
web notes on cosmology (http://www.astro.ucla.edu/~wright/cosmolog.htm)
and was based on calculations published in 1999 by Burles, Nollett and
Turner.